Efficient deployment of wavelength division multiplexed passive optical networks (WDM-PON) is tightly bound to the availability of colourless transceivers, i.e. terminals that can be operated transparently to the WDM channels. As common photo-receivers have an almost flat response in the 1300-1500 nm window, the implementation of colourless terminals is focused on the realization of colourless transmitters.
Directly modulated self-seeded reflective semiconductor optical amplifiers (R-SOA) have been proposed as uplink transmitters for use in WDM-PONs. One proposed colourless transmitter based on R-SOAs exploits a mirror shared by all the optical network units (ONU) in a WDM-PON (Wong et al, Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks, Journal of Lightwave Technology, volume 25, number 1, 2007). For each ONU, the mirror forms a cavity that comprises the R-SOA, the wavelength multiplexer (WDM-MUX) and the drop fibre. Initialization of the self-seeding depends critically on losses introduced by the mirror, the WDM-MUX and the drop fibre. However, the solution proposed by Wong suffers from the problem that the self-seeding is polarization sensitive. Polarization dependency may be a critical parameter if the R-SOAs in the ONUs have a non-negligible Polarization Dependent Gain (PDG); low-cost and uncooled R-SOAs may realistically be expected to exhibit PDG as high as 20 dB. The random polarization fluctuations in the drop fibre may also have a significant effect.